Self-protecting immersion unit



-Se'pt.118, 1934- F. E. FINLAYSON SELF PROTECTING IMMERSION UNIT Filed July 14 1932 Inventor- Frank EFinIagson,

1160M- His/Attorney Patented Sept. 18, 1934 PATENT OFFICE SELF-PROTECTIN G IMMERSION UNIT Frank E. Finlayson, Pittsiield, Mass., assignor to General Electric Company, a corporation of New York Application July 14, 1932, Serial No. 622,435

6 Claims.

My invention relates to electric immersion heaters, and has for its object the provision of an immersion heating unit which is inherently self protecting against heating to destructively high temperatures in the event that the liquid level becomes so low as to partially or completely expose the heating unit.

My invention has particular application to immersion heating units for water, although it oh- 10 viously has application to heaters for various other liquids, such as oils, acids, parafiin, etc. Because of the property inherent in liquids of absorbing and transferring heat, it is the usual practice to construct immersion heaters with a higher wattage output per unit of surface area than would be permissible for operation in the air. This has the advantage of compactness and low cost. It hasthe disadvantage, however, that if the unit is partially or completely exposed to 2 the air while operating at full wattage output capacity it willquickly heat itself to a temperature at which it will be destroyed.

Immersion heaters are very frequently applied to d 'ces in which the water or other liquid is very li ly at some time to reach such a low level as to partially or completely expose the unit. Examples of such devices are steam and hot water sterilizerastills, steam boilers, washing and cleaning machines, coifeev urns, batch type water heaters, etc. When immersion units are applied to such devices they must be watched very carefully; otherwise, replacement of the unit would sooner or later be necessary because of damage from overheating due to low water level.

In carrying out my invention, I provide a metallic casing or sheath in which a heating resistance conductor is enclosed and suitably insulated, preferably by embedding it in powdered heat insulating material. The metallic sheath or casing provides a sealed, heat conductive envelope for the heating unit, whereby the unit is peculiarly adapted for immersion in liquids. I, furthermore, provide a resistance conductor for the heating unit made from a material having a suitably high positive temperature coefficient of resistance and so proportioned with respect to the other parts of the unit that when the liquid level falls so as to partially or completely expose the heating unit, the resulting increase in resistance of the unit lowers the rate of heat generation in the unit to such an extent as to limit the maximum temperature of the unit to a safe temperature which the materials of the unit can withstand without damage. On the other hand,

under normal operating conditions, i. e., complete immersion of the unit, the unit has a desirable high rate of heat generation to facilitate rapid and efficient heating of the liquid.

' For a more complete understanding of my invention, reference should be had to the accom- 00 panying drawing, Fig. 1 of which is a diagrammatic sectional view showing an electric immersion heating unit embodying my invention as applied to a liquid container, while Fig. 2 is a fragmentary enlarged view partially in section 66 showing details of construction of the heating unit.

Referring to the drawing, in carrying out my invention in one form I provide an'outer metallic casing or sheath 10, which is shown as tubular 70 in form and bent in the shape of a hairpin, with its ends suitably secured, asby welding or brazing, to a screw plug 11. By means of this screw plug 11 the unit may be easily applied to a liquid container, such as a water tank 12, by providing 76 a suitable tapped hole inthe wall of the container at the point where the heating unit is to be applied. The hairpin shaped sheathed part of the unit is inserted in the tapped hole and the screw plug 11 screwed tightly in place in the 80 tapped hole to form a leak-proof joint;

As shown in Fig. 2, a sinuous heating resistance conductor 13 is provided in the sheath 10. This resistance conductor is shown as helical in form,and it is secured centrally of'the sheath by 86 means of suitable electrically insulating material 14, such as powdered or granular magnesiumoxide, which surrounds and embeds the resistance conductor. Preferably, the insulating material is suitably compacted in the sheath around 90 the resistance conductor. This compacting operation is preferably performed by reducing the diameter of the sheath, as by rolling or swaging after the unit has been assembled. The sheath, resistance conductor and insulating material are 95. thus combined into a structurally homogeneous unit which may be bent as desired into various shapes to facilitate the application of the unit for the purpose desired or the concentration of heat where desired.

g In order to give automatic protection against damage from overheating in case the unit is partially or completely exposed, I provide a resistance conductor made from a material having a suitably high positive temperature coefllcient of 105 resistance. The increase in resistance within the range of permissible safe temperatures for the materials of the unit should preferably be sufficient to reduce the rate of heat generation of the nuit to form one-half to one-third of its '110 rate under normal operating conditions when entirely submerged. I have found that a nickeliron alloy consisting of approximately 70% nickel and iron has desirable temperature resistance characteristics, and I preferably use this material for the resistance conductor. The resistance conductor is also preferably formed as a round wire. The aforesaid nickel-iron alloy is also resistant to oxidation in air to a very desirably high degree over the temperature rangesof operation for immersion units in many liquids, especially water. I'may also use substantially pure nickel or substantially pure iron. Nickel has the disadvantage of low specific resistance as compared with the nickel-iron alloy. Pure iron has the disadvantage of comparatively rapid oxidation at high temperatures when exposed to air, but by providing a suitable seal for the sheath, iron may be satisfactorily used.

In the application of the unit, a typical example of which is shown in Fig. 1, as long as the unit is operating under normal conditions, that is, completely immersed, the heat is carried away with suillcient rapidity by the liquid to prevent overheating of the unit itself. In fact, the unit is arranged for this normal operating condition. In other words, the resistance conductor is of such length and of such cross-sectional area that when electrically connected to a supply circuit having a suitable voltage, the resistance conductor is heated to such a temperature, and the outer sheath of the unit is heated to such a temperature, although somewhat lower due to the temperature gradient in the resistance material and sheath itself, that heat is supplied to the surrounding liquid by conduotion and convection, i. e.. is taken up by the surrounding liquid, at such a rate that the temperature of no part of the unit is high enough to damage the unit.

As long as the unit remains completely immersed, the conventional resistance conductor having a zero or very small temperature coeflicient of resistance is quite satisfactory. However, due to unforeseen conditions, or lack of attention, the liquid level may become so low as to partially or completeLv expose the heating unit. This is especially true with steam boilers, sterilizers, etc. In such case, the portions of theunit exposed or uncovered rapidly heat to a high temperature, since the steam or other gaseous medium surrounding the exposed portion absorbs heat from the unit at a relatively low rate as compared with the rate at which it is absorbed by the liquid. When the heater is provided with a conventional resistance conductor, the rate of heat generation remains substantially constant as the temperature increases, with the result that the unit rapidly heats itself to a temperature at which oxidation or melting of the sheath and the resistance conductor takes place and the unit is destroyed.

With a resistance conductor having a suitably high positive temperature coefficient of resistance in accordance with my invention, the electrical resistance of the resistance conductor increases with an increase in temperature, and, consequently. the rate of heat generation is correspondin ly reduced. The resistance conductor is, furthermore, so proportioned with respect to the other parts of the unit that a balanced condition of heat generation and heat absorption by the surrounding medium is quickly reached when the unit is partially or completely exposed by an abnormally low liquid level. The temperature of the unit at which this condition of heat equilibrium exists when the unit is completely or partially exposed is limited in accordance-.with myinvention to a safe temperature at which no destructive effect on the heating unit takes place, such as oxidation or melting of the sheath and resistor.

In a typical electric heating unit embodying my invention having a rating in water at 100 C. of 2975 watts, the rate of heat generation when the unit was totally uncovered was reduced to 1350 watts. This unit was provided with a resistance conductor made of an alloy of approximately nickel and 30% iron, and with a sheath made of an alloy of 12% to 14% chromium and the remainder iron. The maximum temperature of the unit when thus operating completely uncovered was less than 500 C. It will be observed that the rate of heat generation was reduced over 50%. I have found that the most severe conditions of operation are encountered when the unit is about one-half covered with water, since with the water at this level the rate of heat generation is still relatively high. The maximum temperature under these conditions of operaton, however, was found to be 600 C. with the unit previously referred to, the temperature decreasing from this point progressively with a decrease in the water level to a maximum of 500 C. when totally uncovered. The materials of the heating unit are selected to withstand this maximum temperature of 600 C., and consequently the unit is absolutely self-protecting for this emergency condition of operation. I have found that a desirable rate of heat generation for units embodying my invention is 30 to 35 watts per square inch of sheath surface with the unit completely immersed in water at C.

This positive temperature coefilcient of resistance feature of the resistance conductor also has the desirable advantage of providing a relatively great rate of heat generation at low temperature whereby rapid heating is obtained during the heating up period. For example, with the heating unit referred to, the rate of heat generation was 3700 watts at 25 C., it being still higher at lower temperatures.

In order to prevent destructive oxidation of the sheath at the maximum temperature of 600 C., I preferably make the sheath of a heat refractory alloy of chromium and iron consisting of approximately 12 %-14% chromium and remainder iron, or chromium, nickel and iron consisting of approximately 18% chromium, 8% nickel and the remainder iron.

The ends of the resistance conductor 13 are connected to terminals (not shown) which are brought out, suitably insulated, through apertures in the plug 11 and connected to insulated connector members mounted on the plug. These connector members are enclosed in a suitable cover 15 secured to the outer end of the plug and having an aperture 16 through which electrical conductors 17 and 18 may be led from the supply source to the terminals. The ends of the sheath are also suitably sealed around the terminals to prevent the entrance of air and moisture. This construction, for example, may be as described and claimed in Patent No. 1,770,824, to- Abbott, dated July 15, 1930. I contemplate also the use of suitable glass seals for the ends of the sheath, such as described and claimed in a copending application of Philip H. Clark, Serial No. 612,497, filed May 20, 1932, and assigned to the same assignee as this invention. It is desirable that an disconnected from the supply source.

eflicient seal be provided to prevent oxidation or the resistance conductor at high temperatures. The seal is especially important when a resistance conductor of iron is used.

In certain heating applications, means responsive to a heating condition in the receptacle, such as the temperature of the liquid medium being heated, or steam pressure, where the heater is applied to a boiler, is provided for controlling the rate 01' heat generation of the heating unit. As shown, I have provided a temperature-responsive bulb 20, which is suitably secured in the receptacle in spaced relation with the heating unit, as shown, above the heating unit, and in position to be immersed in the liquid in the receptacle. This bulb contains a suitable fluid which expands or vaporizes upon an increase in temperature. It is connected through a pipe 21 to the exterior of the receptacle, where a connection is made with a suitable metallic, expansible vessel 22. The vessel 22 is connected to a snap switch 23 of the toggle type, which is shown in closed circuit position and which controls the circuit of an operating coil 24 for the switch 25 in the electrical circuit of the heating unit. A manually operated switch 26 is provided for connecting or disconnecting the heating unit to or from the supply source 27. It will be understood that upon an increase in temperature, producing expansion of the fluid in the bulb, the vessel 22 expands upward and thereby throws the switch 23 to the open position to deenergize the coil 24 and thereby open the switch 25. The heating unit is thus Upon the resulting decrease in temperature of the water or other liquid, the vessel 22 contracts and again closes the switch 23 to reconnect the heating unit to the supply source. A predetermined mean temperature is thereby maintained within predetermined limits.

This temperature-responsive control for the heating unit, while providing for the maintenance of a predetermined temperature in the water, or a predetermined pressure when a pressure-responsive device is provided. gives no protection for low liquid level conditions. For example, if the liquid level becomes low enough from any cause to expose the thermostatic bulb, the temperature of the bulb begins to decrease, thereby assuring that the switch 25 remains closed. The same condition prevails as the water level falls to expose the heating unit, the thermostatic device operating to maintain the heating circuit closed, and thus aflording no protection against overheating. This latter protection, however, is given, as previously explained, by the characteristics of the heating unit itself.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An immersion heating unit for operation normally under conditions of complete immersion in a liquid or like medium, said unit comprising-a metallic sheath made of a metal capable of withstanding in air without detrimental oxidation a predetermined high temperature as compared with the boiling temperature of the liquid or like medium in which the heater is immersed, a heating resistance conductor in said sheath having a resistance at immersion temperatures to give a desirable high rate of heat generation, said resistance conductor being madeof a metal having a high positive temperature coeflicient of resistance to increase its resistance upon an increase in temperature due to partial or complete exposure or the unit to air or the like and thereby decrease its rate of heat generation until a condition of equilibrium between the rates 0! heat generation and heat dissipation is established at a temperature not substantially greater than said predetermined temperature, and electrically insulating material in said sheath surrounding said resistance conductor.

2. An immersion heating unit for water heaters comprising a metallic sheath made of a metal capable oi withstanding in air without detrimental oxidation a predetermined high temperature as compared with the boiling temperature of water, means for mounting said sheath so that it is completely immersed in water under normal conditions of operation, granular electrically insulating material compacted in said sheath, and a resistance heating conductor embedded in said insulating material having such a positive temperature coefllcient of resistance as to reduce the rate of heat generation in said unit by at least per cent upon an increase in temperature of said resistance conductor to a value not substantially greater than said predetermined temperature and thereby limit the temperature of said unit to a safe value in the event of partial or complete exposure of said unit due to low water level.

3. An electric immersion heating unit comprising a leak-proof, heat refractory, heat conductive casing and a resistance heating conductor in said casing made of an alloy oi approximately nickel and 30% iron characterized by a high'tem perature coefficient of resistance such that when the unit is exposed by low liquid level the resulting increase in the temperature of the unit produces an increase in the resistance of said conductor and thereby automatically reduces the rate of heat generation of the unit to prevent temperatures not substantially in excess of 600 degrees C.

4. An electric immersion heating unit comprising a leak-proof heat conductive casing made of a material capable of withstanding a temperature of substantially 600 C. in air without detrimental oxidation, a resistance heating conductor of substantially pure nickel in said casing, powdered heat refractory electrically insulating material in said casing compactedaround said resistance conductor, and means for sealing said resistance conductor in said casing, said resistance conductor being proportioned with respect to the other parts of the unit to give a high rate of heat generation at low liquid temperatures with the unit completely immersed but to automatically decrease the rate of heat generation upon an increase in temperature and thereby limit the temperature of the unit to a maximum value not substantially in excess of 600 C. when the unit is exposed by low liquid level.

5-. An electric immersion heating unit comprising a leak-proof casing made of an alloy containing chromium and iron of such proportions as to withstand a temperature of 600 C. in air without detrimental oxidation, a resistance heating conductor in said casing made of an alloy of approximately 70% nickel and 30% iron, electrically insulating material in said sheath around said resistance conductor, said resistance conductor being proportioned with respect to the other parts of the unit so as to heat said unit when exposed by low liquid level to a maximum temperature not substantially greater than 600 C.

6. An electric heating unit for operation normally under conditions of complete immersion in a liquid medium comprising a casing made of saidresistanceoonductorbeinzproportionedsyith mtotheothervartsottheunitsoastoheat said unit when exposed by low liquid level to a maximum temperature not substantially greater 5 around said resistance conductor, and means for M m c.

sealing said resistance conductor in said casing,

FRANK E. FINLAYBON, 

